Finally got a chance to take a look at the quick start. The art is great and the presentation is clean, and I'm glad it's doing well.
...that said, it seems pretty middle of the road in terms of games within the OSR. It reminds me the most of The Black Hack (unified resolution, 4 classes, streamlined presentation); in fact there seem to be many aspects that are directly inspired (things like movement ranges or explanation of ability tests). But looking at both it's clear that TBH is just more innovative in terms of mechanics (usage die, overloaded encounter tables, monster creation) and more evocative in its random tables (for treasures, traps, poisons, etc). By contrast the Shadowdark character abilities and magic items are a bit meh...lots of +1 or +2 to this or that.
Again, happy it's doing well, but if someone had $30-$50 and wanted to get into the OSR (in particular with a 'modernized' pick up and play ruleset) I'd recommend they start with any number of games that are between $0 and $20 on drivethru: cairn, knave, maze rats, the black hack, worlds without number, index card rpg, sharp swords and sinister spells, etc (some of those games perhaps ironically by people hyping up shadowdark on youtube).